A few people have been saying this. Ignoring the whole thing of Kotaku's response to CAG's contest, what pray-tell 5 seconds of fact checking could Kotaku have done? Contact Microsoft? "Microsoft doesn't comment on rumors or speculation". The blog itself of course didn't say it was fake, and was acting as the primary web source of the information, claiming the source also got other things right in the past (Xbox 360 Elite, HDMI port). The contest itself wasn't something posted on CAG's website that they could just read, it was in a Podcast. Heck, other sites fell for it too (and first), so Kotaku had that to solidify the rumor.
So Kotaku posted it as a rumor, and made clear the exact source of it, so I don't see what mistake Kotaku made in the initial posting of the rumor. They are a gaming blog which prints clearly labelled rumors, and one would hope the average reader would understand the term "rumor"